Scaling Distributed Ledgers and Privacy-Preserving Applications

TL;DR

This thesis advances blockchain scalability, security, and privacy through new protocols and systems like Chainspace, Byzcuit, FastPay, and Coconut, enabling efficient, secure, and privacy-preserving distributed ledger applications.

Contribution

It introduces Chainspace, a scalable sharded ledger supporting smart contracts, and Byzcuit, a secure cross-shard consensus protocol, along with FastPay for low-latency payments and Coconut for privacy-preserving credentials.

Findings

Chainspace achieves scalable, verifiable smart contract execution.

Byzcuit resists replay attacks in sharded ledgers.

FastPay provides low-latency settlement for retail payments.

Abstract

This thesis proposes techniques aiming to make blockchain technologies and smart contract platforms practical by improving their scalability, latency, and privacy. This thesis starts by presenting the design and implementation of Chainspace, a distributed ledger that supports user defined smart contracts and execute user-supplied transactions on their objects. The correct execution of smart contract transactions is publicly verifiable. Chainspace is scalable by sharding state; it is secure against subsets of nodes trying to compromise its integrity or availability properties through Byzantine Fault Tolerance (BFT). This thesis also introduces a family of replay attacks against sharded distributed ledgers targeting cross-shard consensus protocols; they allow an attacker, with network access only, to double-spend resources with minimal efforts. We then build Byzcuit, a new cross-shard consensus protocol that is immune to those attacks and that is tailored to run at the heart of Chainspace. Next, we propose FastPay, a high-integrity settlement system for pre-funded payments that can be used as a financial side-infrastructure for Chainspace to support low-latency retail payments. This settlement system is based on Byzantine Consistent Broadcast as its core primitive, foregoing the expenses of full atomic commit channels (consensus). The resulting system has extremely low-latency for both confirmation and payment finality. Finally, this thesis proposes Coconut, a selective disclosure credential scheme supporting distributed threshold issuance, public and private attributes, re-randomization, and multiple unlinkable selective attribute revelations. It ensures authenticity and availability even when a subset of credential issuing authorities are malicious or offline, and natively integrates with Chainspace to enable a number of scalable privacy-preserving applications.